Moving these utilities to a separate package, so that consumers of this
package don't have to pull in the whole "system" package.
Looking at uses of these utilities (outside of runc itself);
`RunningInUserNS()` is used by [various external consumers][1],
so adding a "Deprecated" alias for this.
[1]: https://grep.app/search?current=2&q=.RunningInUserNS
Signed-off-by: Sebastiaan van Stijn <github@gone.nl>
Sebastiaan van Stijn